The present invention relates to a process for removing impurities from minerals, and more particularly to a process for removing impurities contained in the crystal lattice of minerals.
It is well known that impurities, whether they be desirable or undesirable, may be separated from a mineral or ore by forming suitable salts, i.e. chlorides, bromides, or other haloid salts, which are then separated by vaporizing these salts at relatively moderate temperatures. One of the most widely accepted techniques is the chlorination of an ore or mineral concentrate. Various chlorination methods have been developed for this purpose. In the practice of such chlorination processes, briquettes or pellets of the ore or mineral concentrate are prepared. These briquettes or pellets are then placed in a furnace and are subjected to contact with gaseous chlorine at an elevated temperature, normally above 500xc2x0 C. and usually from about 800xc2x0 C. to about 1200xc2x0 C. to affect chlorination. The chlorides, now containing the impurity, are vaporized and later condensed in suitable condensation equipment. Typical processes for affecting such chlorination are described in U.S. Pat. Nos. 2,681,855, 3,482,964, 4,612,171 and 4,642,133.
Normal separation techniques, such as chlorination, flotation, or other well known techniques for beneficiating minerals cannot remove impurities contained in the crystal lattice of minerals. Impurities such as arsenic may have found their way in the mineral lattice as the mineral was formed. It is desirable to remove such undesirable impurities to thereby enhance the value of the mineral.
The present invention provides a process for the removal of undesirable impurities contained in the crystal lattice of minerals. More particularly, the invention relates to the pyro treatment of a mineral to structurally reorganize the mineral and thereby allow an impurity to freely migrate from the crystal lattice during the structural reorganization to combine with a halogen anion. The anion together with the impurity is thereafter readily separable from the mineral, e.g. by leaching, to thus render the mineral essentially free of the impurity.
Numerous minerals will undergo a structural reorganization, i.e. a reorientation of its crystal lattice, under certain temperature and time conditions. It has been discovered that during this change in the mineral""s structure an impurity in the crystal lattice, which may be very difficult to remove by common mineral purification techniques, becomes free and available which allows the impurity to be mobile and thus removable. The impurity can freely migrate to other sites within the mineral or externally to combine with an anion intimately mixed with the mineral. It has further been discovered that a matrix-forming additive facilitates the migration of the impurity to the anion.
In accordance with the invention, there is disclosed a process for removing impurities contained in the crystal lattice of minerals, comprising the steps of forming a mixture of a mineral containing an undesirable impurity in its crystal lattice, the mineral capable of structurally changing to reorientate its crystal lattice, a halogen anion, and water; heating the mixture to the mineral""s structural reorganization transition temperature; holding the mixture at the transition temperature for a sufficient period of time to allow the impurity to freely migrate from the lattice and combine with the halogen anion; and separating the combined impurity and anion from the mixture to render the mineral essentially free of the impurity. The process is applicable to numerous minerals including most silicates and metal oxides such as fluorspar, spodumene, pyrolusite, braunite, quartz, apatite, bobierrite, evansite, zirconium silicate, feldspars, leurite and anauxite. Also, numerous halogen anions can be employed, such as chlorides, fluorides, bromides and iodides. The preferred halogen anion is a metal chloride such as calcium chloride because it has a relatively high melt temperature and has a high affinity for arsenic. Also, numerous impurities can be removed via the present process such as arsenic and various metal ions such as metal oxides.
Various matrix-forming additives may also be employed with the mixture to facilitate the migration of the impurity from the crystal lattice to the anion. Preferred additives are silicates, added in the form of bentonite, and other clays such as montmorillonite and others of the kaolinite group. It has farther been discovered that a group of organic compounds may also function as the matrix-forming additive. Preferred organic additives include lignosulfonates, starches and starch hydrolyzates.
As noted above, during the mineral""s structural reorganization the impurity combines with the anion to form a halogen salt and this salt is separated to render the mineral essentially free of the impurity. The separating step preferably comprises solubilizing the salt by leaching with an acid such as hydrochloric acid. However, other acids such as nitric acid may also be employed, as well as bases such as sodium hydroxide depending upon the mineral and the impurity to be separated.